Refurbished car batteries are less expensive than new ones. However, as time goes by, refurbished batteries do not accept as much of a charge as they once did, and depending on the state of the internal components, their durability is often much shorter than that of a new battery.
The refurbishment process for batteries generally includes replacement of acid and running a significant reverse current through the system. The purpose of this is to add lead dioxide back to the positive battery plates. This process damages the integrity of the plate, though, shortening the battery life. To make a refurbished battery equivalent to a new one in terms of lifetime and quality, the technician should replace the plates and separators as well as the acid.
Some refurbished car batteries develop what is known as the "memory effect." Over time, as the battery's power wanes, it loses the ability to recharge fully. The battery only remembers that it could power up to a certain point and does not charge past that. This sometimes takes place shortly after purchasing the refurbished battery. In the case of hybrid cars, new batteries are expensive enough to make purchasing a refurbished replacement worth considering. Given that new batteries for gasoline engines are, as of 2015, available for as little as $100 including installation, a refurbished battery for that sort of engine should only be a consideration as an emergency.